Studies on floral determination in the short day plant, Pharbitis nil, and the long day plant, Silene coeli-rosa
I examined the effects of different carbohydrates on floral determination in the short day (SD) plant, Pharbitis nil by excising apices at various times after an inductive 48 h dark period, explanting onto culture media under non-inductive conditions and examining them for flowering 4 weeks later. Cultured apices forming floral organs in non-inductive conditions were determined. The determination time for the sepal, petal and stamen whorls was 1 day whereas it was 5 days for the carpel whorl on medium supplemented with sucrose (2%), on 4-7% (w/v)(-0.29 to 0.51 MPa) it was 3 days but partial replacement of sucrose with mannitol (-0.29 to 0.51 MPa) lenghtened it to 5-6 days; thus, the effect was not caused by the osmotic potential per se. A similar reduction in carpel whorl determination time occurred on 1:1 glucose:fructose and fructose. Again, none of these changes occurred in mannitol treatments. Remarkably, glucose treatment (2, 3 or 6%), resulted in a carpel determination time of one day demonstrating that glucose specified carpels alongside other whorls. DNA replication was measured in the shoot meristems of Silene coeli-rosa (long day (LD) plant) and P.nil during floral determination. The plants were subjected to florally inductive or non-inductive treatments, exposed to tritiated thymidine and apical domes were prepared as fibre autoradiographs. In S. coeli-rosa, replicon size was 10-15um in SD (non-inductive) and 0-5um in LD (inductive) while in P.nil it was 10-15um in the 48 h dark interrupted by red light, 5-10um in continuous light (both non-inductive) but was reduced to 0-5um in the 48 h dark treatment (inductive). Therefore, recruitment of additional initiations points occurred in both a LD and SD plant during floral determination. In the discussion these findings are integrated into unifying models of floral determination.